The present invention relates in general to audio amplifiers and in particular a method and apparatus for obtaining off-line direct audio from a power supply without the use of a relatively high-power amplifier circuit.
Audio circuits are well known in the prior art and generally utilize an audio amplifying circuit to drive the audio speaker. The audio amplifier is required to a be power audio stage. Such power amplifier circuits are illustrated in U.S. Pat. Nos. 4,409,559 and 4,651,112.
A typical class AB audio amplifier has a power supply and the audio amplifier. The cost of these two units together is significant in the manufacture of an audio output device. There are many switching power amplifiers that attempt to avoid the requirement of an audio power amplifier in the circuit.
Such circuits are shown in U.S. Pat. Nos. 4,763,080, 4,517,522, 4,047,120, and 4,453,131.
Class D amplifiers are always switching at fifty percent duty cycle even when no signal is applied. This causes an inherent noise that increases the signal-to-noise ratio when no signal is applied.
It would be advantageous to have a highly efficient effective audio output circuit deriving its energy from a power supply and avoiding the requirement of having an expensive audio amplifier as the final stage.
The present invention relates to an off-line direct audio circuit utilizing the power supply and not requiring a power amplifier output stage. Because all power is delivered directly to the audio speaker from a switching transformer that provides a modulated signal obtaining variable voltages, there is no traditional audio stage like AB or Class D amplifiers.
In the present invention, the audio input signal is applied to an error amplifier with a traditional feedback circuit. The input signal is full-wave rectified after the error amplifier. A pulse-width modulator circuit modulates the signal from the full-wave rectifier and that signal is coupled to a high-powered switching circuit. The high-powered switching circuit comprises a gate-drive unit isolated from the pulse-width modulator with a transformer and an output switching power transformer that transforms modulated high voltage/low current on the input to a modulated low voltage/high current on the output. Because the audio signal has been pulse-width modulated, the signal polarity at the output of the switching power transformer must be determined. Therefore, a zero-crossing detector is utilized to determine the zero crossings of the input audio signal. Those signals are coupled to first and second switches that gate the proper signal polarity outputs of the switching power transformer. The outputs are then filtered with a low-pass filter to remove the modulation signal and generate the audio output signal having sufficient amplitude to drive the speaker. A phase-lag compensation from the feedback circuit is coupled to the input signal and the error amplifier to prevent oscillations of the circuit.
This circuit needs no power audio stage, is extremely efficient due to high voltage on the switching gates, and has a cost about one-half with respect to traditional Class AB power amplifier circuits. Further, when no audio signal is applied, there are no pulses from the pulse-width modulator and thus the signal-to-noise ratio is zero because there is no idle current that flows.
Thus, it is an object of the present invention to provide a direct audio output signal from a power supply.
It is another object of the present invention to pulse-width modulate an input audio signal and to use the pulse-width modulation to drive isolated gates that have a high voltage applied thereto and low current and then utilize an output transformer for switching the high voltage/low current pulse-modulated voltage to a low voltage/high current signal on the output of the transformer that can be properly detected with a switching circuit and then demodulated to provide the audio output signal for driving the speaker.
It is still another object of the present invention to use MOSFETS for the gates utilizing the high voltage and low current inasmuch as the MOSFET devices are more efficient because lower current represents lower losses due to the internal resistance of the MOSFETS.
Further, it is an object of the present invention to provide power MOSFETS that do not switch when there is no input audio signal thus having a zero signal-to-noise ratio when no signal is applied.
It is still another object of the present invention to provide a direct audio from the power supply using an off-line system wherein the power supply is connected to AC line or the AC mains.
Thus, the present invention relates to a method of obtaining off-line direct audio from the power supply comprising the steps of receiving an input audio signal, modulating a power supply voltage with the input audio signal, the power supply voltage being sufficient to drive an audio speaker, coupling the modulated power supply voltage to a filter circuit to recover the audio signal and driving the audio speaker with the recovered audio signal without further amplification.
The invention also relates to apparatus for obtaining off-line direct audio directly from a power supply comprising a terminal for receiving an audio input signal, a voltage source having sufficient voltage of first and second polarity to drive the audio output speaker, a modulator for modulating the voltage from the voltage source with the audio input signals on the terminal, and a filter circuit for recovering the audio signal and providing a driving voltage sufficient to cause the audio signals to be reproduced by the audio output speaker.
Yet, another embodiment of an audio amplifier drive circuit is disclosed that is particularly useful for low voltage applications. The audio amplifier comprises a full wave rectifier for rectifying an input audio signal, a pulse with modulator for producing a signal modulated by the input audio signal, a power amplification stage comprising a pair of transistors in a push-pull configuration, and a voltage step-up switching transformer having an input winding for receiving a power amplified pulse modulated signal generated by the switching transistors. The input winding of the switching transformer has an intermediate tap for receiving a low power supply voltage to bias the switching transistors connected respectively to opposite ends of the input winding.
The audio amplifier drive circuit further includes positive and negative rectifiers connected to an output winding of the switching transformer for positively and negatively rectifying the power amplified pulse width modulated signal, a demodulator for demodulating the positively and negatively rectified pulse width modulated signals, and a switching network for directing the positively rectified pulse width modulated signal to the demodulator in response to a signal polarity detector sensing a positive polarity signal of the input audio signal, and for directing the negatively rectified pulse width modulated signal to the demodulator in response to the signal polarity detector sensing a negative polarity of the input audio signal.
Still, another embodiment of an audio amplifier drive circuit is disclosed that is particularly useful for high power amplification applications, and for driving multiple speakers. This audio amplifier comprises a full wave rectifier for rectifying an input audio signal, a pulse width modulator for producing a pulse width modulated signal from the rectified audio signal, a power amplification stage comprising two pairs of push-pull transistors for respectively producing positive and negative currents through an input winding of a switching transformer. The higher power is achieved by each pair of transistors simultaneously applying the positive and negative rails of a power supply at the input winding of the switching transformer to produce higher currents.
The high power audio drive circuit further includes two positive/negative rectifier pairs connected to the output winding of the switching transistor. One pair of positive/negative rectifier is used to drive a first speaker, another pair drives a second speaker, and both pairs also is used to drive a third speaker connected to the other two speakers in a bridge made configuration. One of the positive/negative pairs is coupled to a switching network that directs a positively rectified pulse width modulated signal to a demodulator in response to a signal polarity detector sensing a positive polarity of the input audio signal, and directs a negatively rectified pulse width modulated signal to the demodulator in response to the signal polarity detector sensing a negative polarity of the input audio detector. The other positive/negative rectifier pair is coupled to a switching network that directs a positively rectified pulse modulated signal to a second demodulator in response to the signal polarity sending a negative polarity of the input audio signal and directs a negatively rectified pulse width modulated signal to the second demodulator in response to the signal polarity detector sensing a positive polarity of the input audio signal.